This invention relates to valve gated injection molding and more particularly to an improved center entry system wherein the valve pin is directly pneumatically operated.
In the past, a variety of different mechanisms have been utilized to drive the valve pins of injection molding systems. The provision of suitable mechanism is considerably easier in multi-cavity systems such as those shown in the applicant's U.S. Pat. Nos. 4,013,393 which issued Mar. 22, 1977 and 4,212,627 which issued July 15, 1980. In these systems having a number of valve pins fed through a manifold arrangement from a common source, the melt flows in from the side and thus there is ample space for actuating mechanism to engage the driven end of the valve pins directly. In fact, in addition to using the lever and cam type drives disclosed in the above mentioned patents, it is even known to use pneumatic and hydraulic piston arrangements in direct alignment with the valve pins.
However, in center entry systems wherein a separate melt passage enters each system in general alignment with the valve pin and the gate, the space problem is much more severe. As may be seen from the applicant's U.S. Pat. Nos. 4,026,518 which issued May 31, 1977, 4,222,733 which issued Sept. 16, 1980, and 4,286,941 which issued Sept. 1, 1981, in systems of this type the melt passage must be offset around the driven end of the valve pin, leaving less room for the actuating mechanism. The mechanical linkage of these previous systems have the disadvantage that it wears considerably under the repeated loading to which it is subjected.
In addition, as described in the applicant's Canadian application Ser. No. 362,420 which was filed Oct. 15, 1980, it has become increasingly advantageous for certain applications for the actuating mechanism to be capable of double acting so that the valve pin may be driven open as well as driven closed. This allows opening to be timed independently of injection pressure which, in addition to avoiding valve pin sticking, in some applications reduces stressing of the material adjacent the gate area. Furthermore, it eliminates the requirement for a pressure shoulder and an enlarged portion of the valve pin which reduces sealing problems.
While pneumatic double acting mechanisms are known, they generally are located outside the melt passage and have the difficulty of transmitting the force to the valve pin, or else are space inefficient in that the melt passage must extend out around the piston. An example of the first type is shown in U.S. Pat. No. 3,677,682 to Putkowski which issued July 18, 1972.